An in-mold expansion molded article obtained by filling a mold with polyethylene resin expanded particles and causing water vapor or the like to heat molding the polyethylene resin expanded particles has characteristics such as an arbitrary property of a shape, a lightweight property, and a heat-insulating property which are advantages of the in-mold expansion molded article.
Various methods for producing polyethylene resin expanded particles are known. Such a method is exemplified by a method (e.g., Patent Literature 1) in which after having been cross-linked, polyethylene resin particles are brought into contact with gaseous carbon dioxide serving as an expanding agent, so as to be impregnated with the carbon dioxide, and thereafter the polyethylene resin particles are water vapor heated, so as to be expanded.
Patent Literatures 2 and 3 disclose polyethylene resin expanded particles obtained as below. Together with carbon dioxide (dry ice), polyethylene resin particles are dispersed into an aqueous dispersion medium. The polyethylene resin particles are heated and pressed, so as to be impregnated with carbon dioxide. Thereafter, the polyethylene resin particles are discharged into a low-pressure region and expanded, so that polyethylene resin expanded particles are obtained. Then, an internal pressure is applied to the polyethylene resin expanded particles thus obtained, and the polyethylene resin expanded particles are steam heated, so as to be further expanded. The following is also disclosed in Patent Literatures 2 and 3. Such a polyethylene resin expanded particle has a cell diameter of not less than 250 μm. In differential scanning calorimetry (DSC), the polyethylene resin expanded particle has two melting peaks that are a low-temperature side melting peak and a high-temperature side melting peak. The high-temperature side melting peak has a heat quantity of 3.5 J/g to 35 J/g.
A method is also known which uses, as an expanding agent, water that is an aqueous dispersion medium mentioned above. In order to cause a polyethylene resin to efficiently absorb water, an ethylene ionomer, as a hydrophilic compound, is mixed with the polyethylene resin (see Patent Literature 4, for example).
Another technique is disclosed in which water or carbon dioxide is used as an expanding agent and polyethylene glycol and/or glycerin is used as a novel hydrophilic compound (see Patent Literature 5, for example).
A method is also known in which, in a case where one-time expansion makes it difficult to obtain expanded particles having a high expansion ratio, expanded particles having a high expansion ratio are obtained by causing expanded particles obtained by one-time expansion to be expanded again.
In differential scanning calorimetry (DSC), polyethylene resin expanded particles disclosed in Patent Literatures 2 through 4 have two melting peaks that are a low-temperature side melting peak and a high-temperature side melting peak.